1. Field of the Invention
The present invention relates to a circuit, especially relates to an ultra-low voltage current reused voltage-controlled oscillator and transconductance-capacitor filter.
2. Description of the Prior Art
With the continued maturation of the Internet of Things (IoT), a huge market has been opening up for short-range ultra-low-power (ULP) wireless connectivity. The IoT market will be close to hundreds of billion dollars (annually ˜16 billions) in 2020. To bring down the hardware cost of such massive inter-connections, sub-GHz ULP wireless products compliant with the existing wireless standard such as the IEEE 802.15.4c/d (ZigBee) will be of great demand, especially for those that can cover all regional ISM bands [e.g., China (433 MHz), Europe (860 MHz), North America (915 MHz) and Japan (960 MHz)]. Together with the obvious goals of small chip area, minimum external components and ultra-low-voltage (ULV) supply (for possible energy harvesting), the design of such a receiver poses significant challenges.
The tradeoffs among multi-band operation, power, area and noise figure (NF) are described in FIG. 1. A multi-band receiver [FIG. 1(a)] can be resorted from multiple low-noise amplifier (LNAs) with shared I/Q mixers and baseband (BB) lowpass filters (LPFs). As such, each LNA and its input matching network can be specifically optimized for one band using passive-LC resonators, improving the NF, selectivity and gain. Although a single wideband LNA with zero LC components is preferred to reduce the die size [FIG. 1(b)], the NF and power requirements of the LNA are much higher. Moreover, when the output noise of the LNA is wideband, more harmonic-folding noise will be induced by its subsequent mixers (under hard switching). All these facts render wideband receivers generally more power hungry than its narrowband counterparts.
In contrast, a wide-range-tunable narrowband RF front-end is of greater potential to realize a multi-band ULP receiver. While sub-GHz passive LC resonators are area hungry, the N-path switched-capacitor (SC) network appears as a prospective alternative to replace them. It behaves as a tunable lossy LC resonator with its center frequency accurately defined by the clock. Inspired by it, present invention introduces a function-reuse RF front-end with signal orthogonality, and a gain-boosted N-path SC network for tunable RF filtering and input impedance matching. External components are avoided, while multi-band operation, stronger RF filtering, smaller physical capacitor size, and lower LO power are concurrently achieved when compared with the traditional designs. Together with a low-voltage current-reuse VCO-filter, the described multi-band receiver exhibits comparable performances with respect to other single-band-optimized designs.